1. Field of the Invention
The present invention relates to a method for producing cyclohexanone oxime. More particularly, the present invention is concerned with a method for producing cyclohexanone oxime, which comprises the steps of: (1) subjecting to an amination reaction a starting material selected from the group consisting of cyclo-hexanol, cyclohexanone and a mixture thereof, thereby obtaining cyclohexylamine, and (2) subjecting the obtained cyclohexylamine to a partial oxidation reaction, thereby obtaining cyclohexanone oxime, wherein a by-product (α) formed in the step (1) and/or a by-product (β) formed in the step (2) are/is recycled to a reaction system of the amination reaction in the step (1). By the method of the present invention, cyclohexanone oxime, which is a useful compound as an intermediate of ε-caprolactam (ε-caprolactam is well-known as a raw material for producing nylon 6 or the like), can be produced with high selectivity, and with other various great advantages in that production of cyclohexanone oxime can be performed, using a simple apparatus, by a simple operation with less consumption of hydrogen, and with no need for use of a difficult reagent, such as a hydroxylamine salt. In the conventional methods for producing cyclohexanone oxime, it is usually required to use a hydroxylamine salt; however, a hydroxylamine salt can be obtained only by a method involving complicated steps, leading to disadvantages.
Further, the method of the present invention is free from problems accompanying the prior art, such as generation of a by-product which is difficult to separate and which adversely affects the quality of ε-caprolactam produced from cyclohexanone oxime, and/or generation of a by-product (e.g., ammonium sulfate) which is of little commercial value. Furthermore, most of the by-products produced in the method of the present invention are recyclable, and with respect to the other by-products, which are not recyclable, most of them are useful compounds, such as cyclohexane and the like, so that generation of wastes can be suppressed to an extremely low level. Thus, the method of the present invention is commercially very advantageous.
2. Prior Art
Conventionally, various methods for producing cyclohexanone oxime have been known. Of such methods, most widely practiced on a commercial scale is a method which consists in producing cyclohexanone from benzene as a starting material by a multi-stage process, and reacting the produced cyclohexanone with a hydroxylamine salt which has been separately produced from ammonia, to thereby obtain cyclohexanone oxime. More specifically, the most widely practiced method involves the following three steps:
(I) producing cyclohexanone from benzene as a starting material,
(II) providing a hydroxylamine salt which has been separately produced from ammonia, and
(III) reacting the cyclohexanone with the hydroxylamine salt to thereby obtain cyclohexanone oxime.
With respect to a representative process for realizing the step (I) of producing cyclohexanone, reference can be made to “Kagakukougaku (Chemical Engineering)”, Vol. 55 (1991), No. 5, page 382, published by The Society of Chemical Engineers, Japan, and “Shokubai (Catalysts and Catalysis)”, Vol. 33 (1991), No. 5, page 341, published by The Catalysis Society of Japan. The most representative process for the step (I) involves oxidation of cyclohexane with air. Another process, which involves hydrogenation of phenol, is also some-times practiced. With respect to the above-mentioned most representative process (i.e., the process involving oxidation of cyclohexane with air), an explanation is made below. This process comprises completely hydrogenating benzene to obtain cyclohexane, subjecting the obtained cyclohexane to oxidation with air to obtain a mixture of cyclohexanol and cyclohexanone, separating the mixture into cyclohexanol and cyclohexanone by distillation, and dehydrogenating the separated cyclohexanol to obtain cyclohexanone.
However, this process has the following disadvantages. First, the process requires a number of steps. Further, in the step of oxidation of cyclohexane with air, for improving the selectivity, it is required to suppress the conversion of cyclohexane to about 3 to 10%, so that the productivity is inevitably lowered. Also, a large amount of energy is needed for recycling the cyclohexane which remains unreacted. Furthermore, the improved selectivity is still unsatisfactorily at a level of from about 73 to 83%. In this process, as by-products, carboxylic acids, alcohols, aldehydes, ketones, ethers, esters, hydrocarbons and the like are generated. In general, these by-products are separated from the desired products (i.e., cyclohexanol and cyclohexanone) and discarded. Of the above-mentioned by-products, water-soluble carboxylic acids, water-soluble lower alcohols and the like can be removed by extraction with water. With respect to the carboxylic acids and esters, which are not water-soluble, such by-products can be removed by saponification with an aqueous alkali solution. Further, most of the other by-products can be removed in the subsequent distillation step. However, with respect to the by-products (such as butyl cyclohexyl ether, n-pentylcyclohexane, cyclohexyl acetate and hexahydrobenzaldehyde) having a boiling point which is very close to that of cyclohexanone or cyclohexanol, it is difficult to remove such by-products, and the presence of these by-products causes a lowering of the quality of the desired compound, i.e., ε-caprolactam. Methods for removing the by-products are disclosed in patent documents, such as Examined Japanese Patent Application Publication No. Sho 60-39656 (corresponding to U.S. Pat. No. 4,661,430), Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-271143 and Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-301858. However, each of the methods disclosed in these patent documents needs a number of cumbersome separation steps and, hence, is not advantageous.
As another method for oxidation of cyclohexane with air, a method is known in which the oxidation is performed in the presence of boric acid. By this method, the conversion and the selectivity are, respectively, improved to a level of from about 12 to 15% and a level of about 90%. However, this method is disadvantageous in that not only is it difficult to handle cyclohexane and a boric acid slurry, but also recycling of these difficult cyclohexane and boric acid slurry is necessary, and the operations therefor are cumbersome.
Further, in the above-mentioned step of dehydrogenation of cyclohexanol, the conversion of cyclohexanol is inevitably limited to at most 70 to 90% due to the equilibrium of the reaction. Also, the boiling point of cyclohexanol as a raw material is very close to that of cyclohexanone obtained as a product, so that a large amount of energy is needed for separating cyclohexanol from cyclohexanone.
With respect to the above-mentioned process for the step (I), which involves hydrogenation of phenol, an explanation is made below. The process involving hydrogenation of phenol has been known since a long time ago. This process comprises producing phenol from benzene, subjecting the produced phenol to hydrogenation at the benzene nucleus in the presence of nickel, palladium or the like as a catalyst to thereby obtain cyclohexanol or cyclohexanone. This process also poses problems. For example, when the production of phenol from benzene is conducted by the cumene process, which is a representative process for production of phenol from benzene, the following disadvantages are caused. The cumene process involves a number of reaction steps. Further, in the cumene process, commercially valuable acetone is coproduced with phenol. Therefore, the cumene process is generally performed using a complicated and expensive apparatus which are needed for separating acetone and phenol from each other, and the demand for and the price of the coproduced acetone influence the production cost of phenol, thereby rendering it difficult to stably supply phenol.
With respect to the above-mentioned step (II) of producing a hydroxylamine salt from ammonia, an explanation is made below. As a representative process for the step (II), there can be mentioned the Raschig process, which is a classical process (see “Kougyou Yuukikagaku (Industrial Organic Chemistry)”, fourth edition, page 287 (1996), translated under supervision of Teruaki Mukaiyama, TOKYO KAGAKU DOZIN CO., LTD., Japan). This process involves four steps. Specifically, the process involves producing ammonium carbonate from ammonia, carbon dioxide and water, synthesizing ammonium nitrite from the produced ammonium carbonate and a mixture of NO and NO2 (wherein the mixture is obtained by oxidation of ammonia with air), reducing the synthesized ammonium nitrate using SO2 to obtain a disulfonate, and hydrolyzing the obtained disulfonate to obtain a sulfuric acid salt of hydroxylamine. This process poses the following problems. The process involves complicated steps. Further, ammonium sulfate, which is of little commercial value, is generated in an amount equimolar to the sulfuric acid salt of hydroxylamine. That is, when the amount of ammonium sulfate by-produced in the oximation is also taken into consideration, the amount of ammonium sulfate by-produced is two moles, relative to one mole of the finally produced cyclohexanone oxime.
Further, with respect to the hydroxylamine sulfate-oxime process (HSO process) and the hydroxylamine phosphate-oxime process (HPO process), each of these processes consists in producing a hydroxylamine salt, and producing cyclohexanone oxime using the produced hydroxylamine salt. These processes also pose various problems, as described below in connection with the above-mentioned step (III).
As a representative process for the step (III), there can be mentioned a process involving oximation of cyclohexanone by the use of a sulfuric acid salt of hydroxylamine (see “Kougyou Yuukikagaku (Industrial Organic Chemistry)”, fourth edition, page 285 (1996), translated under supervision of Teruaki Mukaiyama, TOKYO KAGAKU DOZIN CO., LTD., Japan). The reaction for the oximation is an equilibrium reaction, so that, for advancing the reaction, it is necessary to maintain the pH value at about 7 by adding a certain amount of ammonia to the reaction system. However, when ammonia is added, ammonium sulfate, which is of little commercial value, is inevitably by-produced in an amount equimolar to cyclohexanone oxime.
In connection with the above-mentioned steps (II) and (III), the above-mentioned HSO and HPO processes are explained below. The HSO process involves oxidizing ammonia in the presence of a platinum-containing catalyst to obtain NO, subjecting the obtained NO to reduction with hydrogen in the presence of a platinum -containing catalyst using an ammonium hydrogensulfate/ammonium sulfate buffer solution to produce hydroxylammonium sulfate, and reacting the produced hydroxylammonium sulfate with cyclohexanone (see, for example, U.S. Pat. Nos. 3,941,838 and 4,031,139). The HPO process involves oxidizing ammonia to obtain nitric acid ion, subjecting the obtained nitric acid ion to reduction with hydrogen in the presence of palladium as a catalyst using a phosphoric acid/monoammonium phosphate buffer solution to produce a phosphoric acid salt of hydroxylamine, and reacting the produced phosphoric acid salt of hydroxylamine with cyclohexanone (see, for example, U.S. Pat. Nos. 3,948,988 and 3,940,442). Each of the above-mentioned HSO and HPO processes is advantageous in that the pH value is maintained at a certain level because the buffer solution is allowed to circulate between the cyclohexanone oxime production system and the hydroxylamine salt production system, so that by-production of ammonium sulfate can be prevented. However, the process has the following disadvantages. The process involves a number of reaction steps. Further, high purity raw materials are needed. Furthermore, the step of recovering the catalyst and the step of recycling the buffer solution are complicated. Also, in the whole process, the ammonia-based selectivity for the hydroxylamine salt is as low as about 60%.
Further, the method involving the above-mentioned steps (I) to (III) poses a problem that, for complete hydrogenation of benzene, production of a hydroxylamine salt, and the like, a large amount of hydrogen is needed.
It has been attempted to improve the above-mentioned method involving the steps (I) to (III). For example, with respect to the process for producing cyclohexanone, a process has been proposed which involves subjecting benzene to partial hydrogenation to obtain cyclohexene, hydrating the obtained cyclohexene to obtain cyclohexanol, and subjecting the obtained cyclohexanol to dehydrogenation to obtain cyclohexanone (see Unexamined Japanese Patent Application Laid-Open Specification No. Sho 56-43227 (corresponding to EP 23379)). This process is advantageous not only in that the amount of hydrogen consumed is small as compared to the case of the above-mentioned method involving subjecting cyclohexane to oxidation with air, but also in that it is possible to achieve a carbon-based yield of substantially 100%, wherein the “carbon-based yield” means the total yield of cyclohexanone produced and cyclohexane by-produced in the partial hydrogenation of benzene. However, the process poses various problems. For example, not only does the reaction apparatus used in the step of dehydrogenation of cyclohexanol inevitably become large, but also the necessary energy cost is high, as compared to the case of the above-mentioned process for subjecting cyclohexane to oxidation with air.
As another improved method, there is known a method involving reacting cyclohexanone with ammonia in the presence of hydrogen peroxide to obtain cyclohexanone oxime (see U.S. Pat. No. 4,745,221). This method is advantageous not only in that a difficult reagent (e.g., a hydroxylamine salt) which can be obtained only by a method involving complicated steps is not needed, but also in that ammonium sulfate is not by-produced. However, the method has a problem that hydrogen peroxide, which is expensive, is needed.
On the other hand, methods involving no step of producing cyclohexanone have also been practiced on a commercial scale. As an example of such methods, there can be mentioned a method which involves subjecting benzene to complete hydrogenation to obtain cyclohexane, and reacting the obtained cyclohexane with nitrosyl chloride to obtain a hydrochloric acid salt of cyclohexanone oxime, wherein the nitrosyl chloride is produced by reacting a mixture of NO and NO2 (which mixture is obtained by oxidation of ammonia with air) with sulfuric acid, and then with hydrochloric acid (see “Yuukigousei Kagaku Kyoukaishi (Journal of Synthetic Organic Chemistry, Japan), Vol. 21 (1963), pages 160–163, published by The Society of Synthetic Organic Chemistry, Japan). This method is advantageous in that the number of reaction steps in the method is small as compared to that in the method involving a step of producing cyclohexanone as an intermediate material. However, the method has the following disadvantages. Light is needed for the oximation, so that not only is a large amount of power needed for the oximation, but also maintenance of a mercury lamp or the like used for irradiation of light is cumbersome.
As another method involving no step of producing cyclohexanone, there can be mentioned a method which involves subjecting benzene to complete hydrogenation to obtain cyclohexane, reacting the obtained cyclohexane with nitric acid (which is obtained by oxidation of ammonia) to obtain nitrocyclohexane, and subjecting the obtained nitrocyclohexane to partial hydrogenation to obtain cyclohexanone oxime (see, for example, U.S. Pat. Nos. 3,255,261 and 2,967,200). This method has various problems. For example, the oxidation reaction using nitric acid is required to be performed at a temperature as high as about 150 to 200° C. under a pressure as high as about 3 to 4 MPa, and the apparatus used in the method are greatly corroded. In addition, this method has a danger of explosion. Further, the selectivity for nitrocyclohexane is not satisfactory. Specifically, each of the cyclohexane-based selectivity and the nitric acid-based selectivity is only about 80%. The conversion of cyclohexane is also as low as 15 to 25%. Thus, the productivity is low, and a large amount of energy is needed for recycling the cyclohexane which remains unreacted. Moreover, in the step of producing cyclohexanone oxime by subjecting nitrocyclohexane to partial hydrogenation, the selectivity for cyclohexanone oxime is unsatisfactorily only about 80%.
As apparent from the above, the conventional methods for producing cylohexanone oxime have a serious problem in that the methods inevitably need complicated steps. Therefore, it has been desired to develop a simple and efficient method for producing cyclohexanone oxime, which can be practiced on a commercial scale.